These days, color and density measurement values of printed sheets are often detected by means of a multi-channel measuring method operated in parallel. A multi-channel, measuring method operated in parallel is referred to as an image measuring technique below because it is typically used to detect the measurement data of a whole image or a section of an image on the basis of image pixels. With the age of digital processing, ever increasing importance is being ascribed to the availability of image measurement values and the direct use of digital data from the preliminary processing stage for controlling printing machines. Image measurement values permit efficient quality control of the printed product and are also used for color control and color regulation in the image.
Known methods may be used to detect image measurement values (on the basis of image pixels). One known option is the camera measuring method. It is used in roller printing machines, in digital printing machines and also in sheet offset printing machines as a means of monitoring quality. Line cameras are known, which detect one image line after the other, parallel in sequence. Alternatively, two-dimensional camera systems are used, which detect a limited two-dimensional image field in parallel and compile larger image fields from several measurements with a mechanical offset. Examples of the camera measuring system used in printing machines are the products made by Eltromat GmbH. A specific example is disclosed in patent application EP1213569A2, which describes a camera system specially designed for color measuring systems.
As an alternative to imaging systems, commercially available scanners may be used, in which case the printed sheet is placed on a support and scanned sequentially by a measuring beam. In the simplest approach, the measuring unit of a commercially available flat-bed scanner may be used. Measurement data of better quality can be obtained by using a system specifically optimized for measuring color. Such systems are described in patent U.S. Pat. No. 6,028,682 (≈DE-A 196 50 223) or in this patent application, for example.
If image measurement values are to be used for color applications or density measurements, the image measurement values must be converted into the corresponding variables. The conversion is referred to as colorimetric calibration and can be run in a known manner. A correction matrix is preferably determined by means of a compensating calculation using reference measurement values, which transforms the image measurement values into the desired units (standard color values CIE XYZ or density filter values).
The image measurement values are usually RGB values, multi-filter measurement values (more than 3 measurement values per image pixel) or spectral measurement data (per image point or pixel). As a rule, the colorimetric measuring accuracy of the system is increased, the more different spectral measurement values there are per image pixel or the more accurately the filter functions of the measuring system are adapted to the desired evaluation filters (for example the colorimetric normal observer functions or the density filter functions).
Colorimetric calibration alone is not sufficient for the application of imaging technology in the printing industry. The measuring performance of the system is also affected by process parameters of the printing process and factors dependent on the print medium used.
One known problem is the wet-dry problem which primarily causes difficulties with regard to the measuring technology used in the offset printing method. The printer must be able to test the print quality during the printing operation. At this point in time, the ink applied is still fresh, however. The color coating on the substrate is wet and exhibits a strong sheen. During the drying process, the color coating conforms to the structure of the substrate surface, which reduces the sheen and causes significant changes in the measurement values over time (during the drying phase), especially in the case of mat papers.
The difference between measurement values taken on wet and dry substrate can be reduced using the known polarization filter measuring method. With this measuring method, the sample is illuminated with polarized light and a polarization filter orthogonal to the polarization direction of the illuminating light is used as an analyzer in the collection channel. Orthogonal polarization filters eliminate the component in the measurement light which is reflected from the surface and represents the variable part.
The polarization filter measuring method is primarily used for measuring density and is integrated in commercially available manual measuring systems, such as the spectral photometer, SpectroEye, sold by GretagMacbeth, for example. To date, the polarization filter measuring method has not been used in image-producing color measuring systems for controlling printing processes. The reason for this is that process control systems must be able to take measurements quickly and the orthogonal polarization filters cause a light loss based on a factor of 8 to 10, which has to be compensated by correspondingly longer measuring times, which would be too long for controlling the printing processes. For this reason, patent U.S. Pat. No. 6,028,682 (≈DE-A 196 50 223), for example, describes image measuring systems which are not equipped with polarization filters.
In many applications, however, the measuring system is required to output density values based on the polarization filter measuring method. Furthermore, the polarization filter measuring method offers better linearity of the measurement values as a function of changes in coating thickness and does so when calorimetrically characterizing samples with high densities, such as occur in the case of highly pigmented inks, for example. The polarization filter measuring method would therefore also improve calorimetric regulation of the ink applied by the printing machine possible.
In known measuring systems, such as that described in patent U.S. Pat. No. 6,028,682 (≈DE-A 196 50 223), polarization filter density values are calculated using a correction model using measurement values taken without polarization filters. The correction model operates with fixed parameters. As input variables for the model, the printer can select from a limited number of paper qualities (substrates) only. The relevant correction parameters for these paper qualities are determined on the basis of experiments conducted beforehand. In its simplest form, the correction model corresponds to the subtraction of an offset value from the reflectance value measured without polarization filters. However, the quality obtained on the basis of the correction is not satisfactory. The accuracy of the correction is limited by the large number of different printing substrates with different surface properties. The limited set of typical paper qualities can not emulate this multiplicity. Furthermore, the model is particularly inaccurate with the offset correction for use at high densities and the implementation does not contain sufficient parameters for use with spectral or colorimetric measurement values. Measurement values in the absorption range, at the sides or in the transmission range of a spectrum exhibit various differences depending on whether the measurements are taken with or without polarization filters, which demands a more complex correction model.
The different surface properties of the print samples also cause problems with regard to the measuring geometry. In image measuring methods, it is often not possible to preserve the measuring angle (i.e. 45°/0°-measuring geometry) prescribed for calorimetric methods sufficiently accurately for various reasons. Variations in angle cause differences in the measurement values compared with a color measuring device of standard geometry. However, the differences in the measurement values are also dependent on the paper quality used. Characterizing the measurement differences during manufacture of the device and then running a fixed programmed correction is therefore not good enough for subsequent application using different substrates (paper qualities).
Another problem is the fact that image measuring systems are often designed on a line-oriented basis, such as the system described in U.S. Pat. No. 6,028,682 (≈DE-A 196 50 223), for example. In this case, lighting can be applied from only one angular direction for geometric reasons. In samples with a rough surface, this constraint results in measurement errors induced by direction, depending on how the sample is oriented underneath the measuring device. The color differences induced by direction may be greater than dE*ab=5 on natural paper and the corresponding density variances are greater than 10%. These tolerances are unacceptable for many applications or at least cause problems.